4.4 Hooks

A hook is an external program or script invoked by HTCondor.

Job hooks that fetch work allow sites to write their own programs or scripts,
and allow HTCondor to invoke these hooks at the right moments
to accomplish the desired outcome.
This eliminates the expense of the matchmaking and scheduling provided
by the condor_schedd and the condor_negotiator,
although at the price of the flexibility they offer.
Therefore, job hooks that fetch work allow HTCondor to more easily and directly
interface with external scheduling systems.

Hooks may also behave as a Job Router.

The Daemon ClassAd hooks permit the condor_startd and
the condor_schedd daemons to execute hooks once or on a periodic basis.

Note that standard universe jobs execute different condor_starter and
condor_shadow daemons that do not implement any hook mechanisms.

4.4.1 Job Hooks That Fetch Work

In the past, HTCondor has always sent work to the execute machines by
pushing jobs to the condor_startd daemon, either from the condor_schedd
daemon or via condor_cod.
Beginning with the HTCondor version 7.1.0, the condor_startd daemon now has the
ability to pull work by fetching jobs via a system of plug-ins or
hooks.
Any site can configure a set of hooks to fetch work, completely
outside of the usual HTCondor matchmaking system.

A projected use of the hook mechanism implements what might
be termed a glide-in factory, especially where the
factory is behind a firewall.
Without using the hook mechanism to fetch work,
a glide-in condor_startd daemon behind a firewall
depends on CCB to help it listen and eventually receive
work pushed from elsewhere.
With the hook mechanism, a glide-in condor_startd daemon
behind a firewall uses the hook to pull work.
The hook needs only an outbound network connection to complete
its task,
thereby being able to operate from behind the firewall,
without the intervention of CCB.

Periodically, each execution slot managed by a condor_startd will
invoke a hook to see if there is any work that can be fetched.
Whenever this hook returns a valid job, the condor_startd will
evaluate the current state of the slot and decide if it should start
executing the fetched work.
If the slot is unclaimed and the Start expression evaluates to
True, a new claim will be created for the fetched job.
If the slot is claimed, the condor_startd will evaluate the
Rank expression relative to the fetched job, compare it to
the value of Rank for the currently running job, and decide
if the existing job should be preempted due to the fetched job having
a higher rank.
If the slot is unavailable for whatever reason, the condor_startd
will refuse the fetched job and ignore it.
Either way, once the condor_startd decides what it should do with
the fetched job, it will invoke another hook to reply to the attempt
to fetch work, so that the external system knows what happened to that
work unit.

If the job is accepted, a claim is created for it and the slot moves
into the Claimed state.
As soon as this happens, the condor_startd will spawn a
condor_starter to manage the execution of the job.
At this point, from the perspective of the condor_startd, this claim
is just like any other.
The usual policy expressions are evaluated, and if the job needs to be
suspended or evicted, it will be.
If a higher-ranked job being managed by a condor_schedd is matched
with the slot, that job will preempt the fetched work.

The condor_starter itself can optionally invoke additional hooks to
help manage the execution of the specific job.
There are hooks to prepare the execution environment for the job,
periodically update information about the job as it runs, notify when
the job exits, and to take special actions when the job is being evicted.

Assuming there are no interruptions, the job completes, and the
condor_starter exits, the condor_startd will invoke the hook to
fetch work again.
If another job is available, the existing claim will be reused and a
new condor_starter is spawned.
If the hook returns that there is no more work to perform, the claim
will be evicted, and the slot will return to the Owner state.

4.4.1.1 Work Fetching Hooks Invoked by HTCondor

There are a handful of hooks invoked by HTCondor related to fetching
work, some of which are called by the condor_startd and others by
the condor_starter.
Each hook is described, including when it is invoked, what
task it is supposed to accomplish, what data is passed to the hook,
what output is expected, and, when relevant, the exit status expected.

Hook: Fetch Work

The hook defined by the configuration variable
<Keyword>_HOOK_FETCH_WORK is invoked whenever the condor_startd
wants to see if there is any work to fetch.
There is a related configuration variable called
FetchWorkDelay which determines how long the condor_startd
will wait between attempts to fetch work, which is described in detail
in within section 4.4.1 on
page .
<Keyword>_HOOK_FETCH_WORK is the most important hook in the whole
system, and is the only hook that must be defined for any of the other
condor_startd hooks to operate.

The job ClassAd returned by the hook needs to contain enough
information for the condor_starter to eventually spawn the work.
The required and optional attributes in this ClassAd are identical to
the ones described for Computing on Demand (COD) jobs in
section 4.3.3
on COD Application Attributes,
page .

Command-line arguments passed to the hook

None.

Standard input given to the hook

ClassAd of the slot that is looking for work.

Expected standard output from the hook

ClassAd of a job that can be run.
If there is no work, the hook should return no output.

User id that the hook runs as

The <Keyword>_HOOK_FETCH_WORK hook runs with the same
privileges as the condor_startd. When Condor was started as root,
this is usually the condor user, or the user specified in
the CONDOR_IDS configuration variable.

Exit status of the hook

Ignored.

Hook: Reply Fetch

The hook defined by the configuration variable
<Keyword>_HOOK_REPLY_FETCH is invoked whenever
<Keyword>_HOOK_FETCH_WORK returns data and the condor_startd
decides if it is going to accept the fetched job or not.

The condor_startd will not wait for this hook to return before
taking other actions, and it ignores all output.
The hook is simply advisory, and it has no impact on the behavior of the
condor_startd.

Command-line arguments passed to the hook

Either the string accept or reject.

Standard input given to the hook

A copy of the job ClassAd and the slot ClassAd
(separated by the string ----- and a new line).

Expected standard output from the hook

None.

User id that the hook runs as

The <Keyword>_HOOK_REPLY_FETCH hook runs with the same
privileges as the condor_startd. When Condor was started as root,
this is usually the condor user, or the user specified in
the CONDOR_IDS configuration variable.

Exit status of the hook

Ignored.

Hook: Evict Claim

The hook defined by the configuration variable
<Keyword>_HOOK_EVICT_CLAIM is invoked whenever the condor_startd
needs to evict a claim representing fetched work.

The condor_startd will not wait for this hook to return before
taking other actions, and ignores all output.
The hook is simply advisory, and has no impact on the behavior of the
condor_startd.

Command-line arguments passed to the hook

None.

Standard input given to the hook

A copy of the job ClassAd and the slot ClassAd
(separated by the string ----- and a new line).

Expected standard output from the hook

None.

User id that the hook runs as

The <Keyword>_HOOK_EVICT_CLAIM hook runs with the same
privileges as the condor_startd. When Condor was started as root,
this is usually the condor user, or the user specified in
the CONDOR_IDS configuration variable.

Exit status of the hook

Ignored.

Hook: Prepare Job

The hook defined by the configuration variable
<Keyword>_HOOK_PREPARE_JOB is invoked by the condor_starter before
a job is going to be run.
This hook provides a chance to execute commands to set up the job
environment, for example, to transfer input files.

The condor_starter waits until this hook returns before
attempting to execute the job.
If the hook returns a non-zero exit status, the condor_starter will
assume an error was reached while attempting to set up the job
environment and abort the job.

Command-line arguments passed to the hook

None.

Standard input given to the hook

A copy of the job ClassAd.

Expected standard output from the hook

A set of attributes to insert or update into the job ad. For example,
changing the Cmd attribute to a quoted string changes the executable
to be run.

User id that the hook runs as

The <Keyword>_HOOK_PREPARE_JOB hook runs with the same
privileges as the job itself. If slot users are defined, the
hook runs as the slot user, just as the job does.

Exit status of the hook

0 for success preparing the job, any non-zero value on failure.

Hook: Update Job Info

The hook defined by the configuration variable
<Keyword>_HOOK_UPDATE_JOB_INFO is invoked periodically during the
life of the job to update information about the status of the job.
When the job is first spawned, the condor_starter will invoke this
hook after STARTER_INITIAL_UPDATE_INTERVAL seconds
(defaults to 8).
Thereafter, the condor_starter will invoke the hook every
STARTER_UPDATE_INTERVAL seconds (defaults to 300,
which is 5 minutes).

The condor_starter will not wait for this hook to return before
taking other actions, and ignores all output.
The hook is simply advisory, and has no impact on the behavior of the
condor_starter.

Command-line arguments passed to the hook

None.

Standard input given to the hook

A copy of the job ClassAd that has been augmented with additional
attributes describing the current status and execution behavior of
the job.

The additional attributes included inside the job ClassAd are:

JobState

The current state of the job.
Can be either "Running" or "Suspended".

JobPid

The process identifier for the initial job directly spawned by the
condor_starter.

NumPids

The number of processes that the job has currently spawned.

JobStartDate

The epoch time when the job was first spawned by the condor_starter.

RemoteSysCpu

The total number of seconds of system CPU time (the time spent at
system calls) the job has used.

RemoteUserCpu

The total number of seconds of user CPU time the job has used.

ImageSize

The memory image size of the job in Kbytes.

Expected standard output from the hook

None.

User id that the hook runs as

The <Keyword>_HOOK_UPDATE_JOB_INFO hook runs with the same
privileges as the job itself.

Exit status of the hook

Ignored.

Hook: Job Exit

The hook defined by the configuration variable
<Keyword>_HOOK_JOB_EXIT is invoked by the condor_starter
whenever a job exits, either on its
own or when being evicted from an execution slot.

The condor_starter will wait for this hook to return before
taking any other actions.
In the case of jobs that are being managed by a condor_shadow, this
hook is invoked before the condor_starter does its own optional file
transfer back to the submission machine, writes to the local job event log
file, or notifies the condor_shadow that the job has exited.

Command-line arguments passed to the hook

A string describing how the job exited:

exit The job exited or died with a signal on its own.

remove The job was removed with condor_rm or as the result of
user job policy expressions (for example, PeriodicRemove).

hold The job was held with condor_hold or the
user job policy expressions (for example, PeriodicHold).

evict The job was evicted from the execution slot for
any other reason (PREEMPT evaluated to TRUE in the
condor_startd, condor_vacate, condor_off, etc).

Standard input given to the hook

A copy of the job ClassAd that has been augmented with additional
attributes describing the execution behavior of the job and its
final results.

The job ClassAd passed to this hook contains all of the extra
attributes described above for <Keyword>_HOOK_UPDATE_JOB_INFO, and
the following additional attributes that are only present once a job
exits:

ExitReason

A human-readable string describing why the job exited.

ExitBySignal

A boolean indicating if the job exited due to being killed by a
signal, or if it exited with an exit status.

ExitSignal

If ExitBySignal is true, the signal number that killed the job.

ExitCode

If ExitBySignal is false, the integer exit code of the job.

JobDuration

The number of seconds that the job ran during this invocation.

Expected standard output from the hook

None.

User id that the hook runs as

The <Keyword>_HOOK_JOB_EXIT hook runs with the same
privileges as the job itself.

Hooks are defined in the HTCondor configuration files by prefixing
the name of the hook with a keyword.
This way, a given machine can have multiple sets of hooks, each set
identified by a specific keyword.

Each slot on the machine can define a separate keyword for the set
of hooks that should be used with SLOT<N>_JOB_HOOK_KEYWORD.
For example, on slot 1, the variable name will be
called SLOT1_JOB_HOOK_KEYWORD.
If the slot-specific keyword is not defined, the condor_startd will
use a global keyword as defined by STARTD_JOB_HOOK_KEYWORD.

Once a job is fetched via <Keyword>_HOOK_FETCH_WORK, the
condor_startd will insert the keyword used to fetch that job into
the job ClassAd as HookKeyword.
This way, the same keyword will be used to select the hooks invoked by
the condor_starter during the actual execution of the job.
However, the STARTER_JOB_HOOK_KEYWORD can be defined to
force the condor_starter to always use a given keyword for its own
hooks, instead of looking the job ClassAd for a HookKeyword
attribute.

For example, the following configuration defines two sets of hooks,
and on a machine with 4 slots, 3 of the slots use the global keyword
for running work from a database-driven system, and one of the slots
uses a custom keyword to handle work fetched from a web service.

# Most slots fetch and run work from the database system.
STARTD_JOB_HOOK_KEYWORD = DATABASE
# Slot4 fetches and runs work from a web service.
SLOT4_JOB_HOOK_KEYWORD = WEB
# The database system needs to both provide work and know the reply
# for each attempted claim.
DATABASE_HOOK_DIR = /usr/local/condor/fetch/database
DATABASE_HOOK_FETCH_WORK = $(DATABASE_HOOK_DIR)/fetch_work.php
DATABASE_HOOK_REPLY_FETCH = $(DATABASE_HOOK_DIR)/reply_fetch.php
# The web system only needs to fetch work.
WEB_HOOK_DIR = /usr/local/condor/fetch/web
WEB_HOOK_FETCH_WORK = $(WEB_HOOK_DIR)/fetch_work.php

The keywords "DATABASE" and "WEB" are completely arbitrary, so
each site is encouraged to use different (more specific) names as
appropriate for their own needs.

4.4.1.3 Defining the FetchWorkDelay Expression

There are two events that trigger the condor_startd to attempt to
fetch new work:

the condor_startd evaluates its own state

the condor_starter exits after completing some fetched work

Even if a given compute slot is already busy running other work, it is
possible that if it fetched new work, the condor_startd would prefer
this newly fetched work (via the Rank expression) over the work it
is currently running.
However, the condor_startd frequently evaluates its own state,
especially when a slot is claimed.
Therefore, administrators can define a configuration variable which controls
how long the condor_startd will wait between attempts to fetch new work.
This variable is called FetchWorkDelay.

The FetchWorkDelay expression must evaluate to an integer,
which defines the number of seconds since the last fetch attempt
completed before the condor_startd will attempt to fetch more work.
However, as a ClassAd expression (evaluated in the context of the
ClassAd of the slot considering if it should fetch more work, and the
ClassAd of the currently running job, if any), the length of the delay
can be based on the current state the slot and even the currently
running job.

For example, a common configuration would be to always wait 5
minutes (300 seconds) between attempts to fetch work, unless the slot
is Claimed/Idle, in which case the condor_startd should fetch
immediately:

If the condor_startd wants to fetch work, but the time since the
last attempted fetch is shorter than the current value of the delay
expression, the condor_startd will set a timer to fetch as soon as
the delay expires.

If this expression is not defined, the condor_startd will default to
a five minute (300 second) delay between all attempts to fetch work.

4.4.1.4 Example Hook: Specifying the Executable at Execution Time

The availability of multiple versions of an application leads to
the need to specify one of the versions.
As an example, consider that
the java universe utilizes a single, fixed JVM.
There may be multiple JVMs available, and the HTCondor job may
need to make the choice of JVM version.
The use of a job hook solves this problem.
The job does not use the java universe, and instead uses the
vanilla universe in combination with a
prepare job hook to overwrite the Cmd attribute of the job ClassAd.
This attribute is the name of the
executable the condor_starter daemon will invoke,
thereby selecting the specific JVM installation.

In the configuration of the execute machine:

JAVA5_HOOK_PREPARE_JOB = $(LIBEXEC)/java5_prepare_hook

With this configuration, a job that sets the HookKeyword attribute with

+HookKeyword = "JAVA5"

in the submit description file causes the condor_starter
will run the hook specified by JAVA5_HOOK_PREPARE_JOB
before running this job.
Note that the double quote marks are required to correctly define
the attribute.
Any output from this hook is an update to the job ClassAd.
Therefore, the hook that changes the executable may be

Note that the requirements command ensures that this job
matches with a machine that has JAVA5_HOOK_PREPARE_JOB defined.

4.4.2 Hooks for a Job Router

Job Router Hooks allow for an alternate transformation and/or
monitoring than the condor_job_router daemon implements.
Routing is still managed by the condor_job_router daemon,
but if the Job Router Hooks are specified,
then these hooks will be used to transform
and monitor the job instead.

Job Router Hooks are similar in concept to Fetch Work Hooks,
but they are limited in their scope.
A hook is an external program or script invoked by the condor_job_router
daemon at various points during the life cycle of a routed job.

The following sections describe how and when these hooks are used,
what hooks are invoked at various stages of the job's life,
and how to configure HTCondor to use these Hooks.

4.4.2.1 Hooks Invoked for Job Routing

The Job Router Hooks allow for replacement of the transformation engine used
by HTCondor for routing a job.
Since the external transformation engine is not controlled by HTCondor,
additional hooks provide a means to update the job's
status in HTCondor, and to clean up upon exit or failure cases.
This allows one job to be transformed to just about any other type of job
that HTCondor supports,
as well as to use execution nodes not normally available to HTCondor.

It is important to note that if the Job Router Hooks are utilized,
then HTCondor will not ignore or work around a failure in any hook execution.
If a hook is configured,
then HTCondor assumes its invocation is required and will not
continue by falling back to a part of its internal engine.
For example,
if there is a problem transforming the job using the hooks,
HTCondor will not fall back on its transformation accomplished without the hook
to process the job.

There are 2 ways in which the Job Router Hooks may be enabled.
A job's submit description file may cause the hooks to be invoked with

+HookKeyword = "HOOKNAME"

Adding this attribute to the job's ClassAd causes the condor_job_router
daemon on the submit machine to invoke hooks prefixed with the defined keyword.
HOOKNAME is a string chosen as an example; any string may be used.

The job's ClassAd attribute definition of HookKeyword takes
precedence,
but if not present, hooks may be enabled by defining on the submit machine
the configuration variable

JOB_ROUTER_HOOK_KEYWORD = HOOKNAME

Like the example attribute above,
HOOKNAME represents a chosen name for the hook,
replaced as desired or appropriate.

There are 4 hooks that the Job Router can be configured to use.
Each hook will be described below along with data passed
to the hook and expected output.
All hooks must exit successfully.

Hook: Translate

The hook defined by the configuration variable
<Keyword>_HOOK_TRANSLATE_JOB
is invoked when the Job Router has determined that a job
meets the definition for a route. This hook is responsible for doing the
transformation of the job and configuring any resources that are external to
HTCondor if applicable.

Command-line arguments passed to the hook

None.

Standard input given to the hook

The first line will be the route that the job matched as
defined in HTCondor's configuration files followed by the job ClassAd,
separated by the string "------" and a new line.

Expected standard output from the hook

The transformed job.

Exit status of the hook

0 for success, any non-zero value on failure.

Hook: Update Job Info

The hook defined by the configuration variable
<Keyword>_HOOK_UPDATE_JOB_INFO
is invoked to provide status on the specified routed job
when the Job Router polls the status of routed jobs at intervals
set by JOB_ROUTER_POLLING_PERIOD.

Command-line arguments passed to the hook

None.

Standard input given to the hook

The routed job ClassAd that is to be updated.

Expected standard output from the hook

The job attributes to be updated in the routed job,
or nothing, if there was no update.
To prevent clashing with HTCondor's management of job attributes,
only attributes that are not managed by HTCondor should be output
from this hook.

Exit status of the hook

0 for success, any non-zero value on failure.

Hook: Job Finalize

The hook defined by the configuration variable
<Keyword>_HOOK_JOB_FINALIZE
is invoked when the Job Router has found that the job has completed.
Any output from the hook is treated as an update to the source job.

Command-line arguments passed to the hook

None.

Standard input given to the hook

The source job ClassAd, followed by the routed copy Classad that completed,
separated by the string "------" and a new line.

Expected standard output from the hook

An updated source job ClassAd, or nothing if there was no update.

Exit status of the hook

0 for success, any non-zero value on failure.

Hook: Job Cleanup

The hook defined by the configuration variable
<Keyword>_HOOK_JOB_CLEANUP
is invoked when the Job Router finishes managing the job.
This hook will be invoked regardless of whether the job
completes successfully or not,
and must exit successfully.

Command-line arguments passed to the hook

None.

Standard input given to the hook

The job ClassAd that the Job Router is done managing.

Expected standard output from the hook

None.

Exit status of the hook

0 for success, any non-zero value on failure.

4.4.3 Daemon ClassAd Hooks

Overview

The Daemon ClassAd Hook mechanism is
used to run executables (called jobs) directly from the
condor_startd and condor_schedd daemons.
The output from these jobs is incorporated into the machine ClassAd
generated by the respective daemon.
This mechanism and associated jobs have been identified by various
names, including the Startd Cron, dynamic attributes,
and a distribution of executables collectively known as Hawkeye.

Pool management tasks can be enhanced by using a
daemon's ability to periodically run executables.
The executables are expected to generate ClassAd attributes as their
output; these ClassAds are then incorporated into the machine ClassAd.
Policy expressions can then reference dynamic attributes (created
by the ClassAd hook jobs) in the machine ClassAd.

Job output

The output of the job is incorporated into one or more ClassAds when
the job exits.
When the job outputs the special line:

- update:true

the output of the job is merged into all proper ClassAds,
and an update goes to the condor_collector daemon.

As of version 8.3.0, it is possible for a Startd Cron job (but
not a Schedd Cron job) to define multiple ClassAds, using the
mechanism defined below:

An output line starting with '-'
has always indicated end-of-ClassAd. The '-' can now be
followed by a uniqueness tag to indicate the name of the ad that
should be replaced by the new ad. This name is joined to the name
of the Startd Cron job to produced a full name for the ad.
This allows a single Startd Cron job to return multiple
ads by giving each a unique name, and to replace multiple ads by
using the same unique name as a previous invocation. The optional
uniqueness tag can also be followed by the optional keyword
update:<bool>, which can be used to override the
Startd Cron configuration and suppress or force immediate
updates.

In other words, the syntax is:

- [name] [update: bool]

Each ad can contain one of four possible attributes to control
what slot ads the ad is merged into when the condor_startd sends
updates to the collector. These attributes are, in order of highest
to lower priority (in other words, if SlotMergeConstraint
matches, the other attributes are not considered, and so on):

SlotMergeConstraint expression: the current ad is merged
into all slot ads for which this expression is true. The expression
is evaluated with the slot ad as the TARGET ad.

SlotName|Name string: the current ad is merged into all slots
whose Name attributes match the value of SlotName
up to the length of SlotName.

SlotTypeId integer: the current ad is merged into all ads
that have the same value for their SlotTypeId attribute.

SlotId integer: the current ad is merged into all ads
that have the same value for their SlotId attribute.

For example, if the Startd Cron job returns:

Value=1
SlotId=1
-s1
Value=2
SlotId=2
-s2
Value=10
- update:true

it will set Value=10 for all slots except slot1 and slot2.
On those slots it will set Value=1 and Value=2
respectively. It will also send updates to the collector immediately.